1. Field of the Invention
The invention relates generally to a laser ranging and detecting system in which a coherent beam of light is transmitted, reflected from a target, received and processed to extract information concerning the target.
More particularly, the invention relates to a laser ranging and detection system in which the coherent beam of light is scanned through a predetermined pattern by a rotating wedge. The system may be of the doppler detection type in which the frequency of the received signal beam is compared with a portion of the transmitted signal beam to determine the velocity of targets.
2. Description of the Prior Art
In laser scanning systems of the present type, the received signal beams are processed by mixing them with a portion of the transmitted signal beam termed the local oscillator signal. Because of the finite transit time of the transmitted beam to the target and back, the received signal beams strike the rotating wedge at a different angle than that at which they were transmitted. This change in angle between transmitted and received beams causes the position of the received beam as it is focused upon the detector to vary. The position upon the detector varies with both the range to the target and the scanning rate. Because both of these parameters may be varied during a system scanning operation, fixed compensation cannot be used.
One prior solution was simply to slow down the rate of rotation of the scanning wedge to a speed such that the local oscillator signal and received signals were in sufficient alignment to produce a processable signal. Unfortunately, the maximum permissible scanning rate under such circumstances is too slow to be of use in most practical surveillance and reconnaissance applications.